Power MOSFETs include metal wires deposited to connect source elements to one another and to connect drain elements to one another, usually in parallel. Typically, a metal film is deposited over a dielectric layer on a semiconductor wafer. The metal film is patterned and etched to leave the required metal wires. The metal wires make contact with various active areas (e.g., the drain areas, source areas, and/or gates) using vias. Vias are holes previously etched in the dielectric layer then filled with a conductor such as tungsten (e.g., using chemical vapor deposition or CVD). For more complicated connections, additional layers of metal may be separated by additional insulating layers and connected to one another by further vias therethrough. U.S. Pat. No. 8,937,351 entitled “Power MOS Transistor with Improved Metal Contact” relates to MOSFETs and is hereby incorporated by reference in its entirety.
FIG. 1A is a drawing showing a cross section taken through a portion of a prior art MOSFET 100 having a N-type epitaxial layer 110, a P-type base 120 disposed therein, and an N-type source implant 130 implanted into the base 120. MOSFET 100 also includes gates 140 and contacts 150 connecting base 120 and source implant 130. Contact 150 comprises a conductor filling a via extending through layer 160 and source implant 130 into the base 120.
FIG. 1B shows a side view along line 1B from FIG. 1A, another cross-section of MOSFET 100 taken perpendicular to the cross-section of FIG. 1A. As may be seen in FIG. 1B, each contact 150 makes a connection with base 120 along the bottom 156 of the contact 150 and along the lower portion 154 of the sides of the contact 150. Connection between contact 150 and source implant 130 is at side portions 152 of contact 150 along the thickness of source implant 130.